1. Field of the Invention
This invention relates to compounds which synergize the behavioral responses of insects induced by their attractant pheromones. These compounds may be used in combination with pheromones and insect control measures such as toxicants or traps. The compounds also extend the life of pheromone-baited traps and decrease the amount of pheromone needed.
2. Description of the Prior Art
Insect-produced volatiles, e.g., pheromones, and host plant odors, e.g., kairomones, may facilitate location of conspecifics for mating and orientation to acceptable host plants for feeding and oviposition. It is known in several insect species, especially bark beetles, that pheromones and plant odors, such as monoterpenes, may act in synergy, each enhancing the attraction of the other [Borden, in Insect Communication, ed. T. Lewis, Academic Press, NY, page 123 (1984)]. For example, response of Dendroctonus brevicomis Lec. to its aggregation pheromone is enhanced by the host monoterpene, myrcene [Bedard et al., Science 164: 1284-1285 (1969)]. Response of the boll weevil to its pheromone is enhanced by a mixture of five monoterpenes [Chang et al., Southwest. Entomol. 11: 233-241 (1986)]. The tripartite blend of a host monoterpene and two component aggregation pheromone used in chemoorientation by D. brevicomis was thought to increase specificity of the pheromone respone [Birch, in Chemical Ecology of Insects, eds. W. J. Bell and R. T. Carde, Chapman and Hall Ltd., London, pages 331-353, (1984)].
Green leaf volatiles, six-carbon alcohols and aldehydes (e.g., 1-hexanol, trans-2-hexen-1-ol , cis-3-hexen-1-ol and their corresponding aldehydes, hexanal, trans-2-hexenal, and cis-3-hexenal) are known to occur in green plants as a product of oxidative degradation of plant lipids [Visser et al., J. Chem. Ecol. 5: 13 (1979)]. Previous studies have shown these compounds to be active as host plant attractants [Visser et al., Entomol. Exp. Appl. 24: 538-549 (1978); Katsoyannos et al., Entomol. Exp. Appl. 35: 71-74 (1984)], enhancers of other host plant odors [Guerin et al., J. Chem. Ecol. 9: 843-861 (1988), indicators of fruit is ripeness [Engel et al., J. Agric. Food Chem. 36: 1003-1006 (1988); Light et al., Proc. XVIII Internat. Congr. Entomol. Abstr., 213 (1988)] or defensive secretions [Hamilton et al., J. Chem. Ecol. 11: 1399-1409 (1985)].
The attractant pheromone of the boll weevil was previously identified and shown to consist of four oxygenated monoterpenoid components [Tumlinson et al., Science 166: 1010-1012 (1969)]. The detection of green leaf volatiles by insects [Visser, Annu. Rev. Entomol. 31: 121 (1986)], including the boll weevil, has been previously shown [Dickens, J. Chem. Ecol. 10: 1759 (1984)].
The smaller European elm bark beetle, Scolytus multistriatus (Marsham), is a vector of Dutch elm disease which threatens elm tree populations where it occurs. Multilure (the aggregation pheromone mixture of xcex1-multistriatin, methyl heptanol, and cubebene) [Pearce et al., J. Chem. Ecol. 1: 115-124 (1975)] trapping is used primarily as a means of removing portions of resident S. multistriatus populations from a given area.
The cotton plant itself and an aqueous extract of cotton leaves have been shown to increase responses of boll weevils in laboratory bioassays to grandlure, the pheromone of the boll weevil [Hardee et al., J. Eco. Ent. 64(6): 1454 (1971)]; however, among the more than 250 volatile components of cotton [Hedin et al., Annu. Rev. Phytochem. 10: 271 (1976)], the specific compounds responsible for the increased effect were not isolated or identified.
I have now discovered that members of the green leaf volatile complex, especially trans-2-hexen-1-ol, cis-3-hexen-1-ol, 1-hexanol, hexenal, and trans-2-hexanal are effective synergists for insect pheromones.
In accordance with this discovery, it is an object of the invention to provide new positions for attracting insects as an aid to insect control measures.
A further object of the invention is to provide new means to synergize the effect of insect pheromones.
A further object of the invention is to provide a means for increasing the effectiveness of insect traps for monitoring or suppressing insect populations.
Other objects and advantages of this invention will become obvious from the ensuing description.
I have found that green leaf volatiles enhance the attractant pheromone response of the boll weevil, Anthonomus grandis Boh., the smaller European elm bark beetle, Scolytus multistriatus (Marsham), and the Mediterranean fruit fly, Ceratitis capitata Weid.
The importance of olfaction in the beavior of insects is well known. Insect-produced volatiles, e.g., pheromones, and host plant odors may facilitate location of conspecifics for mating and orientation to acceptable host plants for feeding and oviposition. Pheromones, which may be attractive alone, may be enhanced or synergized by host plant odors which show little attraction when presented alone.
A synergist is herein defined as a material that enhances the activity of other materials, so that the overall activity of the mixture is greater than the sum of the individual components.
An effective amount of a synergist is herein defined as an amount of synergist which in combination with the appropriate insect pheromone will attract a statistically greater number of insects than the sum of the pheromone and the synergist alone.
An effective synergist for an attractant pheromone is useful in several ways:
1. A synergist improves population monitoring with the pheromone not only by increasing the attractiveness of the pheromone but also by modifying the sex ratio of the insects attracted.
2. A synergist improves attractiveness of the pheromone, thus facilitating trap-out strategies.
3. An inexpensive synergist reduces the cost of insect control, since its addition to the pheromone in traps decreases the quantity of costly pheromone needed and extends the longevity of the attractive bait.
4. Pheromone traps for the boll weevil, which currently must be placed in the cotton field, will be more cost effective if a synergist makes them attractive when placed outside the cotton field.
The boll weevil is a serious pest of cotton throughout the southeastern United States, Arizona, and Central and South America. It was introduced from Mexico in the late 1800""s and spread northward and eastward to areas where cotton was grown. Economic losses caused by this insect devastated cotton production in this region and required the use of large quantities of insecticides. The cost of insecticides for the control of insects on cotton and corn exceeds that for any other crops. Although the use of insecticides in the control of cotton insects is widespread, the use of currently available chemicals poses a threat both to man and his environment. Furthermore, insects have developed resistance to certain insecticides.
With the identification of the boll weevil pheromone in 1969, a tool was available to monitor boll weevil populations for directing insecticide applications and evaluating control measures. The pheromone could also potentially be used in trap-out strategies. However, the pheromone is composed of four oxygenated monoterpenes that are expensive and subject to oxidation, and no practical synergist is currently available for the pheromone.
The boll weevil is narrowly oligophagous, feeding primarily on cotton, Gossypium hirsutum L., and closely related Malvaceae. Once male boll weevils locate their host plant, feeding ensues, and they release an attractant pheromone in their frass. This pheromone is used in the Boll Weevil Eradication Program for both census and evaluation of control and/or eradication efforts. I have discovered a potent class of chemicals that, when released with the boll weevil pheromone, enhances its attraction, increases the longevity of pheromone-baited traps, and decreases the amount of pheromone needed.
This class of compounds, known as green leaf volatiles, includes six-carbon alcohols and aldehydes which are products of oxidation of plant lipids. The six-carbon chains may be saturated or unsaturated.
Specifically, trans-2-hexen-1-ol, cis-3-hexen-1-ol, and 1-hexanol, when added to traps in the field, both enhance attraction of the pheromone and prolong the attractiveness of pheromone-baited traps. Green leaf volatiles occur in all green plants and will serve to synergize or enhance behavioral responses of other insects to their attractant pheromones.
Study of individual olfactory cells from males and females by electrophysiology revealed that specific cells were responsive to stimulation with grandlure, whereas other cells were activated by volatiles emanating from a crushed cotton square. No cell responded to both stimuli.
Many cells activated by volatiles emanating from the crushed cotton square could also be activated by green leaf volatiles, especially trans-2-hexen-1-ol (Tale I). Not only were these cells unresponsive to the attractant pheromone but they also did not respond to monoterpenes or sesquiterpenes found in the cotton plant.
Results of competitive experiments with a triangular array of traps showed that addition of trans-2-hexen-1-ol to grandlure significantly increased its attraction relative to grandlure alone (Table II). Trans-2-hexen-1-ol, 1-hexanol, and cis-3-hexen-1-ol alone were relatively inactive when presented in competition with the other treatments.
Results of tests with a four trap array showed that trans-2-hexen-1-ol at a dosage as low as 0.1 mg in combination with grandlure (0.8 mg) increased trap capture when in competition with grandlure (0.8 mg) alone (P less than 0.10; paired t-test) (Table III). When trans-2-hexen-1-ol was increased to 1.0 mg, enhancement of grandlure (0.8 mg) was even more apparent (P less than 0.01; paired t-test). Similarly, both 1-hexanol (1 mg) and cis-3-hexen-1-ol (1 mg) were effective synergists for grandlure (0.8 mg) when in competition with grandlure (0.8 mg) alone (P less than 0.05; paired t-test). However, when 1-hexanol and trans-2-hexen-1-ol were each presented in combination with grandlure (0.8 mg) in a competitive experiment, trans-2-hexen-1-ol in combination with grandlure (0.8 mg) was more effective than the combination with 1-hexanol. When the aldehyde, trans-2-hexenal, was released with grandlure vs grandlure alone, no significant difference was noted in trap captures. However, this was the only dosage of this compound tested, and subsequent gas chromatographic analyses showed that the aldehyde
had in part been degraded by the field conditions in which it was released. Cis-2-hexen-1-ol, a questionable member of the general green leaf volatile complex, also showed no synergism. In these competitive experiments, comparisons between numbers of weevils captured are appropriate only between treatments tested simultaneously.
In competitive field tests in cotton fields, our results showed that the synergist, trans-2-hexen-1-ol, at 1 mg, 10 mg, and 100 mg in combination with grandlure at 1 mg captured more weevils of both sexes than grandlure alone at 1 mg (P less than 0.01; paired t-test) (Tale IV). It seed that the synergist was especially effective with lower populations, such as occurred in fields containing trans-2-hexen-1-ol at 1 mg and 100 mg.
An experiment to determine the longevity of attractiveness of traps baited with grandlure (1 mg) and grandlure (1 mg)+trans-2-hexen-1-ol revealed that the number of boll weevils captured in each field with grandlure alone decreased from the first to the second wk (Table V). Similarly, the total number of weevils captured by grandlure+trans-2-hexen-1-ol at the lowest dosage tested (1 mg) also decreased during this period. However, when grandlure was combined with trans-2-hexen-1-ol at 10 mg and 100 mg, trap capture actually increased with increasing dosages of the synergist from wk #1 to wk #2. It should also be noted that in each case the percent of weevils captured by grandlure+trans-2-hexen-1-ol vs grandlure creased from wk #1 to wk #2, and more weevils were captured with grandlure+trans-2-hexen-1-ol than with grandlure alone over the duration of the test.
The results of my experiments in the cotton field were surprising and indicate that trans-2-hexen-1-ol not only enhances trap capture when presented with grandlure but also extends the life of grandlure-baited traps and decreases the amount of grandlure needed.
Although numerous components of the attractant produced by Mediterranean fruit fly males have been chemically identified [Baker et al., J. Chem. Soc. Chem. Comm., 824-825 (1985); Jang et al., Entomol. Exp. Appl. (in press)], the active constituents have not been fully resolved. Therefore, the pheromonal odor of calling males was tested in competition with male odor plus a combination of green leaf volatiles in a laboratory flight tunnel. Green leaf volatiles increased the number of landings made by female flies on the odor source relative to male odor alone, as well as increasing the mean maximum number of flies on the spheres [Table VI(c)]. In fact, the major green leaf volatile of the blend, trans-2-hexenal, alone enhanced response of female medflies to male odor. Similar to the boll weevil and smaller elm bark beetle, little or no response was elicited by the green leaf volatiles alone.
When 1-hexanol or hexanal or a mixture of 1-hexanol and hexanal were released with multilure, the multicomponent aggregation pheromone of S. multistriatus [Pearce, supra], captures of bark beetles in traps were increased relative to the pheromone alone [Table VI(b)]. Neither the boll weevil nor the smaller elm bark beetle were attracted to traps baited with the green leaf volatiles alone (Tables II and VI).
The potency of these synergized pheromone compositions dictates that they be applied in conjunction with a suitable inert carrier or vehicle as known in the art. Of particular interest are those which are agronomically acceptable. Alcohols, glycols, ketones, esers, aqueous mixtures, and solid carriers such as clays or cellulose are illustrative of suitable carriers. The synergized pheromone compositions may be used in a number of ways; for example, in combination with pesticides to kill the insects or in traps to monitor population changes. Other formulations and methods of use will be obvious to skilled artisans.
The synergized pheromone compositions encompassed herein are effective in attracting a variety of organisms. Without desiring to be limited thereto, pests of particular interest known to be susceptible to treatment are agronomically important insects, especially the boll weevil, Anthonomus grandis BOH; the European elm bark beetle, Scolytus multistriatus; and the Mediterranean fruit fly, Ceratitis capitata. 
A typical synergized pheromone opposition contemplated by this invention for use in an inert trap comprises trans-2-hexen-1-ol 0.1 mg to 100 mg, preferably 1.0 mg; grandlure 0.1 mg to 10 mg, preferably 0.8 mg, combined with a suitable amount of an inert carrier.
Without desiring to be bound by any particular theory of operation, it is believed that the green leaf volatiles act by stimulating certain specific olfactory cells in the insect antenna.